Here the problem:
the connection between deb1 and deb2 works perfectly,
but not the one between deb1 and deb3. When I shutdown deb2 then the connection between deb1 and deb3 start to works. It looks like deb1 is not able to listen at the same time deb2 and deb3.

What am I doing wrong?

I actually followed what is written in this guide (look at the section 4.2 Guest-only network: "When you want to use more than three guest OSes, just connect to the one. Use different macaddr.")
and this article